radix tree test suite: put definition of bitmap_clear() into lib/bitmap.c

In tools/ directory, function bitmap_clear() is currently only used in
object file tools/testing/radix-tree/xarray.o.

But instead of keeping a bitmap.c with only bitmap_clear() definition in
radix-tree's own directory, it would be more proper to put it in common
directory lib/.

Sync the kernel definition and link some related libs, no functional
change is expected.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
CC: Matthew Wilcox <willy@infradead.org>
CC: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>